The present invention relates to a fuel cell in which power generation is made by using chemical reaction between a fuel and an oxidant and an electronic device including such a fuel cell.
Currently, as an electrochemical device with which a new style different from a primary battery and a secondary battery is able to be proposed, a fuel cell in which power generation is made by using chemical reaction between a fuel and an oxidant has attracted attention. The fuel cell has a structure in which an electrolyte is arranged between an anode electrode (fuel electrode) and a cathode electrode (oxygen electrode). The fuel is supplied to the anode electrode side, and the oxidant is supplied to the cathode electrode side. Thereby, chemical energy of the fuel is effectively converted to electric energy. In the fuel cell, since power generation is made by using the chemical reaction between the fuel and the oxidant, electric power is able to be continuously extracted by supplying oxygen in the air as the oxidant and continuously resupplying the fuel from outside.
As the fuel cell, various types of fuel cells have been already proposed or experimentally produced, and part thereof is practically used. Specially, since a Polymer Electrolyte Fuel Cell (PEFC) in which a proton conductive polymer film is used as an electrolyte operated at comparatively low temperature such as about from 30 deg C. to 130 deg C. both inclusive without an electrolytic solution, the PEFC is able to be thinned, and is suitable as an electric power source for a mobile electronic device.
Further, as a fuel thereof, various materials such as hydrogen and methanol are able to be used. Specially, since a liquid fuel such as methanol has a higher density than that of gas and is easily stored, the liquid fuel is taken as a hopeful fuel for a fuel cell included in a small electronic device. In particular, since a Direct Methanol Fuel Cell (DMFC) in which methanol is directly supplied to the anode electrode to initiate reaction does not need a reformer for extracting hydrogen from a fuel, the DMFC is advantageous to realizing a small and thin device.
In such a DMFC, as a method of supplying methanol to the anode electrode, a liquid supply type DMFC and a vaporization supply type DMFC have been proposed. Of the foregoing, in the liquid supply type DMFC, a liquid fuel (methanol aqueous solution) is directly supplied to the anode electrode. In the liquid supply type DMFC, since the liquid fuel is directly contacted with the anode electrode, methanol crossover in which methanol is moved from the anode electrode side to the cathode electrode side through the electrolyte film is easily generated. In particular, in the case where highly concentrated methanol is used, voltage reduction is caused, and usage efficiency of methanol is significantly lowered. In such a case, reaction is not able to be effectively promoted unless the fuel concentration is decreased. However, if the fuel concentration is decreased, in addition to lowering of energy density, excessive water reaches the cathode electrode, and flooding phenomenon is more easily generated. Further, on the anode electrode contacted with the methanol solution, carbon dioxide (CO2) is generated by oxidation reaction. Thus, bubbles of carbon dioxide are adhered onto the electrode, and supplying the methanol solution is inhibited. Thus, the output is lowered or becomes unstable.
Meanwhile, in the vaporization supply type DMFC, based on moisture concentration equilibrium between both electrodes, water generated in the cathode electrode is inversely diffused to the anode electrode side, and thereby water consumed in the anode electrode is able to be resupplied. Thus, a highly-concentrated methanol aqueous solution is able to be used, and moisture in the electrolyte film is able to be retained by self-humidification, and accordingly proton conductivity necessary for power generation is able to be maintained in the electrolyte film. Therefore, usage efficiency of methanol is improved, and a high energy density is able to be retained. In the past, as a vaporization fuel supply method, a method of naturally vaporizing a fuel in a fuel tank, a method of using capillary force (for example, Patent Documents 1 and 2) and the like have been proposed. In addition, a technology of providing a pump (fuel supply section) for pumping a liquid fuel from a storage tank and supplying the liquid fuel to the power generation side and a fuel vaporization section for vaporizing the liquid fuel supplied by the fuel supply section has been proposed.
[Patent Document 1] Japanese Patent No. 3413111
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 6-188008